Storage.hs

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnliftedFFITypes #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE BangPatterns #-}
-- | A perilous implementation of thread-local storage for Haskell.
-- This module uses a fair amount of GHC internals to enable performing
-- lookups of context for any threads that are alive. Caution should be
-- taken for consumers of this module to not retain ThreadId references
-- indefinitely, as that could delay cleanup of thread-local state.
--
-- Thread-local contexts have the following semantics:
--
-- - A value 'attach'ed to a 'ThreadId' will remain alive at least as long
--   as the 'ThreadId'. 
-- - A value may be detached from a 'ThreadId' via 'detach' by the
--   library consumer without detriment.
-- - No guarantees are made about when a value will be garbage-collected
--   once all references to 'ThreadId' have been dropped. However, this simply
--   means in practice that any unused contexts will cleaned up upon the next
--   garbage collection and may not be actively freed when the program exits.
--
-- Note that this implementation of context sharing is
-- mildly expensive for the garbage collector, hard to reason about without deep
-- knowledge of the code you are instrumenting, and has limited guarantees of behavior 
-- across GHC versions due to internals usage.
module Control.Concurrent.Thread.Storage 
  ( 
    -- * Create a 'ThreadStorageMap'
    ThreadStorageMap
  , newThreadStorageMap
    -- * Retrieve values from a 'ThreadStorageMap'
  , lookup
  , lookupOnThread
    -- * Update values in a 'ThreadStorageMap'
  , update
  , updateOnThread
    -- * Associate values with a thread in a 'ThreadStorageMap'
  , attach
  , attachOnThread
    -- * Remove values from a thread in a 'ThreadStorageMap'
  , detach
  , detachFromThread
    -- * Update values for a thread in a 'ThreadStorageMap'
  , adjust
  , adjustOnThread
    -- * Monitoring utilities
  , storedItems
    -- * Thread ID manipulation
  , getThreadId
#if MIN_VERSION_base(4,18,0)
  , purgeDeadThreads
#endif
  ) where

import Control.Concurrent
import Control.Concurrent.Thread.Finalizers
import Control.Monad ( when, void, forM_ )
import Control.Monad.IO.Class
import Data.Maybe (isNothing, isJust)
import Data.Word (Word64)
import GHC.Base (Addr#)
import GHC.IO (IO(..), mask_)
import GHC.Int
#if MIN_VERSION_base(4,18,0)
import GHC.Conc (listThreads)
#endif
import GHC.Conc.Sync ( ThreadId(..) )
import GHC.Prim
import qualified Data.IntMap.Strict as I
import qualified Data.IntSet as IS
import Foreign.C.Types
import Prelude hiding (lookup)
import Unsafe.Coerce (unsafeCoerce#)

foreign import ccall unsafe "rts_getThreadId" c_getThreadId :: Addr# -> CULLong

numStripes :: Word
numStripes = 32

getThreadId :: ThreadId -> Word
getThreadId (ThreadId tid#) = fromIntegral (c_getThreadId (unsafeCoerce# tid#))

stripeHash :: Word -> Int
stripeHash = fromIntegral . (`mod` numStripes)

readStripe :: ThreadStorageMap a -> ThreadId -> IO (I.IntMap a)
readStripe (ThreadStorageMap arr#) t = IO $ \s -> readArray# arr# tid# s
  where
    (I# tid#) = stripeHash $ getThreadId t

atomicModifyStripe :: ThreadStorageMap a -> Word -> (I.IntMap a -> (I.IntMap a, b)) -> IO b
atomicModifyStripe (ThreadStorageMap arr#) tid f = IO $ \s -> go s
  where
    (I# stripe#) = fromIntegral $ stripeHash tid
    go s = case readArray# arr# stripe# s of
      (# s1, intMap #) ->
        let (updatedIntMap, result) = f intMap 
        in case casArray# arr# stripe# intMap updatedIntMap s1 of
             (# s2, outcome, old #) -> case outcome of
               0# -> (# s2, result #)
               1# -> go s2
               _ -> error "Got impossible result in atomicModifyStripe"
          
-- | A storage mechanism for values of a type. This structure retains items
-- on per-(green)thread basis, which can be useful in rare cases.
data ThreadStorageMap a = ThreadStorageMap 
  (MutableArray# RealWorld (I.IntMap a))

-- | Create a new thread storage map. The map is striped by thread
-- into 32 sections in order to reduce contention.
newThreadStorageMap 
  :: MonadIO m => m (ThreadStorageMap a)
newThreadStorageMap = liftIO $ IO $ \s -> case newArray# numStripes# mempty s of
  (# s1, ma #) -> (# s1, ThreadStorageMap ma #)
  where
    (I# numStripes#) = fromIntegral numStripes

-- | Retrieve a value if it exists for the current thread
lookup :: MonadIO m => ThreadStorageMap a -> m (Maybe a)
lookup tsm = liftIO $ do
  tid <- myThreadId
  lookupOnThread tsm tid

-- | Retrieve a value if it exists for the specified thread
lookupOnThread :: MonadIO m => ThreadStorageMap a -> ThreadId -> m (Maybe a)
lookupOnThread tsm tid = liftIO $ do
  m <- readStripe tsm tid
  pure $ I.lookup threadAsInt m
  where 
    threadAsInt = fromIntegral $ getThreadId tid

-- | Associate the provided value with the current thread.
--
-- Returns the previous value if it was set.
attach :: MonadIO m => ThreadStorageMap a -> a -> m (Maybe a)
attach tsm x = liftIO $ do
  tid <- myThreadId
  attachOnThread tsm tid x

-- | Associate the provided value with the specified thread. This replaces
-- any values already associated with the 'ThreadId'.
attachOnThread :: MonadIO m => ThreadStorageMap a -> ThreadId -> a -> m (Maybe a)
attachOnThread tsm tid ctxt = 
  updateOnThread tsm tid (\prev -> (Just ctxt, prev))

-- | Disassociate the associated value from the current thread, returning it if it exists.
detach :: MonadIO m => ThreadStorageMap a -> m (Maybe a)
detach tsm = liftIO $ do
  tid <- myThreadId
  detachFromThread tsm tid

-- | Disassociate the associated value from the specified thread, returning it if it exists.
detachFromThread :: MonadIO m => ThreadStorageMap a -> ThreadId -> m (Maybe a)
detachFromThread tsm tid = liftIO $ do
  let threadAsInt = getThreadId tid
  updateOnThread tsm tid (\prev -> (Nothing, prev))

-- | The most general function in this library. Update a 'ThreadStorageMap' on a given thread,
-- with the ability to add or remove values and return some sort of result.
updateOnThread :: MonadIO m => ThreadStorageMap a -> ThreadId -> (Maybe a -> (Maybe a, b)) -> m b
updateOnThread tsm tid f = liftIO $ mask_ $ do
  -- ^ We mask here in order to ensure that the finalizer will always be created
  (isNewThreadEntry, result) <- atomicModifyStripe tsm threadAsWord $ \m -> 
    let (resultWithNewThreadDetection, m') = 
          I.alterF 
            (\x -> case f x of
              (!x', !y) -> ((isNothing x && isJust x', y), x')
            ) 
            (fromIntegral threadAsWord)
            m
     in (m', resultWithNewThreadDetection)
  when isNewThreadEntry $ do
    addThreadFinalizer tid $ cleanUp tsm threadAsWord
  pure result
  where 
    threadAsWord = getThreadId tid

update :: MonadIO m => ThreadStorageMap a -> (Maybe a -> (Maybe a, b)) -> m b
update tsm f = liftIO $ do
  tid <- myThreadId
  updateOnThread tsm tid f

-- | Update the associated value for the current thread if it is attached.
adjust :: MonadIO m => ThreadStorageMap a -> (a -> a) -> m ()
adjust tsm f = liftIO $ do
  tid <- myThreadId
  adjustOnThread tsm tid f

-- | Update the associated value for the specified thread if it is attached.
adjustOnThread :: MonadIO m => ThreadStorageMap a -> ThreadId -> (a -> a) -> m ()
adjustOnThread tsm tid f = liftIO $ do
  atomicModifyStripe tsm threadAsWord $ \m -> (I.adjust f (fromIntegral threadAsWord) m, ())
  where 
    threadAsWord = getThreadId tid 

-- Remove this context for thread from the map on finalization
cleanUp :: ThreadStorageMap a -> Word -> IO ()
cleanUp tsm tid = do
  atomicModifyStripe tsm tid $ \m -> 
    (I.delete (fromIntegral tid) m, ())

-- | List thread ids with live entries in the 'ThreadStorageMap'.
-- 
-- This is useful for monitoring purposes to verify that there
-- are no memory leaks retaining threads and thus preventing
-- items from being freed from a 'ThreadStorageMap' 
storedItems :: ThreadStorageMap a -> IO [(Int, a)]
storedItems tsm = do
  stripes <- mapM (stripeByIndex tsm) [0..(fromIntegral numStripes - 1)]
  pure $ concatMap I.toList stripes
  where
    stripeByIndex :: ThreadStorageMap a -> Int -> IO (I.IntMap a)
    stripeByIndex (ThreadStorageMap arr#) (I# i#) = IO $ \s -> readArray# arr# i# s

#if MIN_VERSION_base(4,18,0)
-- | This should generally not be needed, but may be used to remove values prior to GC-triggered finalizers being run from the 'ThreadStorageMap' for threads that have exited.
purgeDeadThreads :: MonadIO m => ThreadStorageMap a -> m ()
purgeDeadThreads tsm = liftIO $ do
  tids <- listThreads
  let threadSet = IS.fromList $ map getThreadId tids
  forM_ [0..(numStripes - 1)] $ \stripe ->
    atomicModifyStripe tsm stripe $ \im -> (I.restrictKeys im threadSet, ())
#endif